Preliminary studies have been completed evaluating the effects of changes in perfusion media (Tyrode) pH on the electrophysiologic properties of Purkinje fibers. Studies were done utilizing changes in bicarbonate concentration (pH 7.2, 18 mM HCO3-; pH 7.4, 25 mM HCO3-; pH 7.6, 42 mM HCO3-) and in the pCO2/pO2. Over this test range, in studies with changes induced by HCO3-, no significant differences were seen in the following parameters: dV/dt of phase O, rest potential, total amplitude, action potential duration at 50, 75 and 95 percent of full recovery as well as the effective refractory period. Subsequently studies have been performed following exposure to lidocaine, 1 x 10 to the minus 5th power M, and procaine amide, 30 mg/L. The experiments with lidocaine showed no significant differences in drug effect at any test pH. Lidocaine produced at all pH ranges, a shortening of effective refractory period and action potential duration with no significant effects on resting potential, amplitude or dV/dt. Following exposure to procaine amide, depression of dV/dt was observed as well as shortening of action potential duration. These changes were also similar regardless of the pH of the Tyrode solution. Exposure to Tyrodes with varied pCO2/pO2 produced minimal action potential and effective refractory period shortening at higher pH's. Initial studies with cat papillary muscle are to begin immediately with a similar format to evaluate the effects of changes in pH of antiarrhythmic drugs on electrophysiologic and mechanical properties of cardiac muscle.